This invention relates to a process for preparing a sizing agent for paper in the form of an aqueous dispersion with a high free rosin content, from resins based upon reinforced and/or non-fortified rosins and water, with the use of an anionic dispersing agent. This invention also relates to aqueous dispersions useful as paper sizing agents, and to their use in the sizing of paper.
Paper sizing agents based upon fortified rosin in the form of aqueous dispersions with a high free resin content have been known for a long period of time. For example, German Patent No. 1,131,438 describes aqueous dispersions containing fortified rosins, such as adducts of rosin and .alpha.,.beta.-unsaturated carbonyl compounds, such as fumaric acid, maleic acid, and the like. However, in order to achieve a certain stability in such dispersions, it has been necessary to add fatty acids and/or naphthenic acids in addition to the fortified rosin. Furthermore, the presence of a protective colloid, such as casein, is required as well. However, the concomitant use of casein is accompanied by several disadvantages. Primarily, casein is a valuable, expensive albumin product, and secondarily, dispersions made with the use of casein have only limited stability in storage and tend to show a precipitate after a certain storage time. Furthermore, the use of casein is accompanied by troublesome odors.
Various possibilities for the preparation of dispersions from free casein have been previously explored. In this respect, one may refer to German Pat. No. 1,958,965 which describes a process for the preparation of a fortified rosin sizing in accordance with which a solution is first made of the rosin based material and an organic solvent immiscible with water, which solution is then emulsified in water. After homogenizing, the organic solvent immiscible with water is, in essence, completely removed. The process is relatively complex and furthermore, working with organic solvents may pose potential health or environmental problems. Reclamation of the organic solvent is difficult and waste water contaminated with the organic solvents give rise to considerable ecological problems.
It has also been attempted to obtain stable dispersions of fortified rosin by using many various dispersing agents, such as salts of alkylaryl sulfonic acids, sulfonated higher fatty alcohols, sulfonated castor oil, resin soaps, and the like. Frequently, the use of these dispersing agents requires the concomitant use of additional protective colloids, such as casein, and special processing methods are often required to obtain a dispersion with a fine distribution. In addition, the resulting dispersions often have a stability of short duration and cannot be stored for any length of time. Besides numerous compounds with the sulfo substituents derived from succinic acid, German Pat. No. 2,627,943 also discloses compounds with are ethylene oxide adducts. Thus, the compound which is used to form the dispersion is obtained when a maleate is used as an initial material that has been esterified with an ethylene oxide adduct of an alkanol with at least 6 carbon atoms.
Although a large number of paper sizing agents in the form of aqueous dispersions containing high free resin content derived from rosin based material are already known, there still exists a need for a paper sizing agent of this type with improved characteristics which can be obtained through simple processing, while demonstrating suitable economic benefits.
An object of the present invention is therefore to make available a process, according to which the economic preparation of such dispersions will be possible in a simple manner, which process will also lead to dispersions having excellent stability, suitable for surface sizing, as well as internal sizing.
It is also an object of the present invention to make available a process by which dispersions may be prepared that can be stored for an extended period of time and that contain a high concentration of solid substances based upon rosin, leading to paper with a high degree of sizing that can be processed without difficulty.
A further object of the present invention is to make available sizing agents for paper that are compatible with the usual additives employed in the sizing of paper.